This invention relates generally to temperature sensing techniques, and more particularly to a self-verifying device and method for temperature measurement and control.
Many important properties of chemistry, physics, thermodynamics and heat transfer can only be determined by accurately measuring temperature. The measurement of temperature is fundamental to most modern industries. However, errors in temperature measurement can result in energy inefficiencies, creating scatter in product quality, shortening plant life, and limiting plant safety. The effects of temperature measurement error accumulate over days, weeks, and months of operation and are a significant cost to many industries.
One problem frequently encountered with conventional temperature measurement results from drift. Drift in temperature sensors can present a major roadblock to an industry""s effort to create new product types or increase energy efficiency. For example, new gas turbine engines for the aircraft industry must operate at hotter temperatures in order to achieve increased fuel efficiency. Temperature sensors are required to monitor the hotter gas flow temperatures for the new engines in order to operate properly. In general, hotter temperatures tend to degrade sensors more rapidly and cause more rapid aging. Aging increases the probability of erroneous temperature readouts.
In fact, jet airliners must be taken out of service in order to re-check the calibration of thermocouples that monitor the temperature within gas turbine engines. Gas turbine temperature measurement becomes essential to establishing fuel combustion ratios. The cost to take airliners out of service for such check-up is considerable, but necessary, because there previously has not existed a convenient method of confirming the calibration of temperature sensors while they remain installed in a gas turbine engine. The requirement for calibration checks for new gas turbine engines that will operate at hotter temperature will compound the costs that result from temperature sensor decalibration. Furthermore, high maintenance and product quality costs are caused by temperature sensor decalibration that is common to a wide variety of modern industrial and transportation applications.
Sometimes the consequences of poor temperature measurement can result in catastrophic failures. One such accident destroyed the Three Mile Island-Two nuclear power plant, which could have been prevented by a quick and accurate measurement of reactor core temperatures. During such accident, reactor operators were unable to determine whether the reactor core was overheating. Ignorance over core temperature allowed the plant operators to make an erroneous conclusion about the state of the reactor core. The control decision error caused the loss of the Three Mile Island plant, with some total cost estimates running as high as $3 billion.
As another example, a recent DC-10 airplane crash was traced to failure of an engine support strut. The strut failure was subsequently traced to improper temperature control in a metal annealing process. Temperature sensors that controlled the annealing process had decalibrated, and the operator was unable to detect temperature drift in sensor readout.
The importance of accurate and reliable temperature measurement to modern processing and transportation industries is well documented by the above and similar examples that demonstrate the vulnerability of modern industry to temperature sensor drift. Furthermore, a number of techniques exist for monitoring temperature, but existing techniques each have associated problems.
For example, there exist a number of schemes and techniques for monitoring the intensity of heat by measuring temperature. One early technique entailed the monitoring of thermal expansion in order to sense a temperature. Such physical phenomena forms the basis for liquid-in-glass thermometers. Several other techniques involve electrical transduction which is employed to sense temperature. Among these are resistive, thermoelectric, semiconductive, optical and piezoelectric detectors. Temperature measurement involves the transmission of a small portion of an object""s thermal energy to a sensor, the sensor functioning to convert that energy into an electrical signal. For the case where a contact sensor is used, the contact sensor is placed inside or on an object, with heat conduction taking place through an interface between the object and a probe. The probe warms up or cools down, exchanging heat with the object. Through careful design of a probe, the measurement site will not be disturbed significantly and error is minimized by appropriate sensor design via correct measurement techniques.
One problem associated with a significant number of such measurement techniques occurs when temperatures have to be measured under tough or hostile environments. Such tough or hostile environments can involve strong electrical, magnetic or electromagnetic fields, or very high voltages which make measurements either too susceptible to interferences, or too dangerous for an operator. Hence, one technique for solving such problems is to use non-contact techniques for measuring temperature. However, non-contact techniques do not work in many environments. Additionally, there exist contact sensors which can sense temperature in a hostile environment, such as thermocouples which measure resistive coupling of different materials when exposed to a temperature environment.
For the case of a thermocouple, comprising a thermo-electric contact sensor, at least two dissimilar conductors are used to make a sensor. A number of different thermocouples are known for use with different applications such as TypeT, TypeJ, TypeE, TypeK, Types R and S, and TypeB. Depending on the temperature and/or chemical environment encountered, a suitable thermocouple can be selected. However, one problem with thermocouple sensors results from xe2x80x9cdriftxe2x80x9d, as discussed above, which can adversely affect accuracy by causing measurement errors.
Another problem results from the aging of temperature sensors which can result in increases in temperature system error. Gregory K. McMillan, Advanced Temperature Control, published by The Instrument Society of America (1995), discusses mathematical methods to estimate the magnitude of error that has accrued on an aged temperature sensor. However, mathematical estimates of error are not adequate to correct for temperature sensor drift, in most industrial processes.
Modern industrial processes are carried out at ever-increasing temperatures. Such rise in operating temperature requires the measurement of temperature in service conditions that are increasingly corrosive or otherwise hostile to measurement instruments. For these reasons, temperature sensors increasingly age, or otherwise degrade, while operating under the combined stresses of modern service conditions. Such aging process causes errors in calibration to creep into the readout of temperature measurement instruments. Therefore, plant operators are required to detect drift in temperature sensor readouts, and to correct for such drift, or replace sensors that are known to have decalibrated. Replacement of such sensors usually requires a scheduled shutdown of the process. Accordingly, the detection of drift in temperature sensor readout is generally not easy. Furthermore, the scheduled shutdown of a process is undesirable and costly.
For example, thermocouple drift is usually caused by trace contaminants which migrate into the thermal element wires, changing their composition. The alteration in composition likewise changes signal output for a given temperature. Changes in such signal output cause an error in temperature readout. Other causes of thermocouple drift include breach of the outer protection sheath, which usually results in deterioration of the electrical insulation. Hence, errors are caused from shunting of the signals being generated by the thermal elements. Drift, or deviations in signal output, occur in most thermocouples as they age under the stresses of modern industrial conditions. Similar conditions cause drift in Resistance Temperature Devices (RTDs), or resistance devices.
As a further example, optical sensors commonly encounter drift problems. Such drift problems include the formation of films on optical surfaces that distort the frequency spectrum of the signal monitoring temperature. Additionally, other impairments may preclude an accurate determination of temperature, such as inability to correct for unknowns in emissivity of optical targets. For example, two-color devices often only partially correct for errors in emissivity.
Accordingly, the inability to verify in situ the calibration of temperature sensors also creates a major obstacle to the introduction of automated control systems in modern industrial plants. Such modern plants rely heavily on trained operators in order to detect when temperature sensors may be out of calibration. The operators determine that sensors are out of calibration by referring to secondary parameters that relate to temperature, such as increases in product defects, temperature measurements by nearby sensors, and increase in fuel consumption, or by other means, most of which are considered xe2x80x9cblack magicxe2x80x9d, or xe2x80x9cartxe2x80x9d. According to presently existing temperature measurement technologies, automated temperature control loops are incapable of verifying temperature sensor accuracy. Such automated control loops are required to take the temperature sensor readout and treat it as xe2x80x9cgospelxe2x80x9d (assumed accurate), performing control functions that presume that the incoming readouts from temperature sensors remain accurate. Since all temperature sensors on the market today age when subjected to modern hostile service environments, the usefulness of fully automated temperature control loops is limited for many applications. Accordingly, improvements are needed to determine whether such temperature sensors remain accurate, and to provide for a useful control signal that is capable of being reliably utilized with fully automated temperature control loops for one of any of a number of applications.
Another technique for measuring temperature involves the use of optical non-contact temperature detectors comprising radiation thermometry. Radiation thermometry is also recognized under the name of pyrometry. Such a non-contact technique for measuring temperature utilizes a sensing element that is responsive to electromagnetic radiation in the infrared wavelength range. Such a sensing element must be fast, predictable and strong, responsive to thermal radiation, and have a good long-term stability. However, such optical thermal detectors are only suitable for use in limited temperature environments.
U.S. Pat. No. 5,183,338 discloses yet another temperature measurement system that is configured to overcome the limited effective temperature range typically encountered with optical sensor systems. A black body sensor is combined with a photoluminescent sensor to provide an optical apparatus capable of accurately measuring temperatures over a wide range. The black body sensor is more suitable for use with high temperature ranges, whereas the photoluminescent sensor is suitable for measuring temperatures over a lower temperature range. Such an optical probe is configured so as to provide both a black body sensor and a photoluminescent sensor. Signal processing is utilized to switch between the two sensors in the region where an overlap exists between the two temperature ranges. A limited ability to calibrate temperature measurements might be made within such overlapped temperature ranges by utilizing the photoluminescent sensor to calibrate measurements made by the black body sensor. However, such overlap range is relatively small and ineffective and does not necessarily provide an accurate calibration over the high temperature range and the low temperature range, outside the overlap region. Therefore, self-verification of temperature measurement is not possible over typical desired temperature ranges.
In summary, many modern industries face a fundamental problem in all temperature measurements; namely, there is no convenient means to verify the accuracy of temperature measurements while temperature sensors remain in an industrial use application. Temperature sensors are calibrated under laboratory conditions at the manufacturer""s plant prior to shipment. Once such temperature sensors have been installed in an application, it is not generally convenient or possible to re-check the calibration of such sensors during operation. Therefore, removal of temperature sensors for verification of calibration in a laboratory is not usually practical. Furthermore, such sensor removal is never economical.